I've found that size doesn't matter much (as the actress said to the bishop) - in that you can get away with fitting a very slightly larger O ring from your box of assorted pound shop O rings. Remember that neoprene is flexible and compressible so size isn't critical to the micron - as long as it's not too small (said the actress).
Also, don't forget that the old O ring will have dried out and shrunk slightly over time and exposure to ethanol / petrol, causing it to leak, so replace with one very slightly larger to ensure it seals snuggly. Job done.